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Two decades ago, Space Shuttle Columbia rocketed into orbit carrying one of the most unusual experiments on record: the reflight of the Tethered Satellite System (TSS-1R). It was intended to demonstrate the “electrodynamics” of a conducting tether in an electrically charged region of Earth’s atmosphere, known as the ionosphere, and it was hoped that such

‘Broken at the Boom’: 20 Years Since STS-75 (Part 2) by Ben Evans March 6, 2016, 8:00 am Spectacular view of Columbia’s payload bay and aft compartment, during STS-75. Photo Credit: NASA, via Joachim Becker/SpaceFacts.de Two decades ago, Space Shuttle Columbia rocketed into orbit carrying one of the most unusual experiments on record: the reflight of the Tethered Satellite System (TSS-1R). It was intended to demonstrate the “electrodynamics” of a conducting tether in an electrically charged region of Earth’s atmosphere, known as the ionosphere, and it was hoped that such technology could lead to sophisticated power-producing sources for spacecraft. As noted in yesterday’s history article, the mission began smoothly on 22 February 1996, and within days the seven-man STS-75 crew—Commander Andy Allen, Pilot Scott “Doc” Horowitz, Payload Commander Franklin Chang-Díaz, Mission Specialists Jeff Hoffman, Claude Nicollier, and Maurizio Cheli, and Payload Specialist Umberto Guidoni—was in position to deploy the satellite. The intention was that, as the tether neared its maximum length of 12.7 miles (20.5 km), the deployment rate would have been gradually reduced. However, shortly after deployment commenced, things started to go awry. “It was within 1 km of its final length,” Hoffman told the NASA oral historian, “at which point we were going to put on the brakes and just let it sit there and start all the experiments. I was recording this huge arc in the tether through the camera, when I started to see little ripples in the tether.” To Hoffman’s eyes, it reminded him of the tether jam on STS-46—the first TSS mission, back in July-August 1992—and a horrible sense of déjà vu dawned on him. However, at 8:29:35 p.m. EST on 26 February, at a tantalisingly closet-to-target distance of 12.2 miles (19.6 km), it became clear that the tension was due to something else: the tether had not jammed, but snapped. The shocked astronauts recorded video footage of the incident, and the breakage appeared to have taken place near the top of the mast. “The tether has broken at the boom!” Hoffman radioed urgently. “It is going away from us.” In fact, the tether and the satellite were accelerating away from the shuttle at a rate of about 415 miles (670 km) during each 90-minute orbit. By the morning of the 27th, it was trailing Columbia by 3,000 miles (4,830 km), flying some 30 miles (50 km) “above” the shuttle. After winding the remaining 30 feet (10 meters) of tether back into the mechanism, the astronauts retracted the mast to its original configuration. Having sustained a tether jam on STS-46, it was hoped that the reflight would generate greater success for the unique Tethered Satellite System. Photo Credit: NASA, via Joachim Becker/SpaceFacts.de It was fortuitous that the breakage occurred close to the top of the mast, rather than further outward, close to the satellite. “If it breaks at the bottom, it will fly away from you and you’re not in any danger,” said Hoffman, “but if it breaks at the top you’ve got 20 km of tether coming snapping back at you. We had practiced for that eventuality in the simulator. You’ve got to then cut the tether at the bottom and fly away from it.” Although nearly 24 hours of electrodynamic measurements had been lost, the $154 million reflight was far from a total failure. Already, when the satellite was less than 3.7 miles (6 km) from the mast, the Deployable Core Experiment (DCORE) recorded its first data. This experiment was mounted in the payload bay on a Multi-Purpose Equipment Support Structure (MPESS), and its task was to control the flow of electric current in the tether using a pair of electron guns. Before the break, its first performance run had successfully generated a current of 480 milliamps from the electrical charge that had collected on the satellite’s surface. This was about 200 times greater than the levels obtained during the first TSS flight on STS-46 in July 1992. Other experiments in the payload bay continued to function in support of the satellite and tether until as late as 6 March. “We did get a lot of good data during the deploy,” Hoffman told journalists during a space-to-ground news conference. Currents measured during the deployment were at least three times higher than predicted in analytical models. Voltages as high as 3,500 volts were developed across the tether, achieving current levels of 480 milliamps. It was also possible for researchers to study the interaction of gas from the satellite’s thrusters with the ionosphere. A first-ever direct observation of an ionized shockwave around the satellite—impossible to study or model in laboratories on Earth—was also accomplished. Moreover, as the satellite and its trailing tether sped through the ionosphere, it was possible to continue investigations in spite of the fact it was no longer physically linked to the shuttle. It did not detract from the disappointment, however. “If you don’t ever get your nose bloodied,” said STS-7...

‘Broken at the Boom’: 20 Years Since STS-75 (Part 2) - AmericaSpace